Aircraft designed to operate in rarefied atmosphere typically employ an air cycle environmental control system to cool, filter, pressurize, and otherwise condition cabin air. In most installations, compressed ambient air, provided by either the engine compressor section, the auxiliary power unit, or both, is expanded in an air cycle turbomachine, providing a cool, fresh, air supply for the cabin. The costs of this cool, fresh, air supply are twofold. First, due to the size and number of components required for their assembly, these systems can appreciably increase the gross weight of the aircraft. Second a considerable amount of energy, stored in the compressed, ambient, supply air, is needed to satisfy the cooling requirements of even an average-sized aircraft. In an industry faced with increasing fuel costs and heightened environmental concerns, considerable effort is made to reduce, without sacrificing overall system performance, both the weight and energy requirements of these systems.
Since compressed ambient air is readily available, it is a convenient source of power for airborne environmental control systems. In most systems, the compressed, ambient air is passed through a heat exchanger cooled by air from outside the aircraft, lowering its temperature to around ambient air temperature. To further lower the temperature of the compressed ambient air, it is expanded in a turbine. If the temperature of the expanded air falls below its dew point, any water vapor entrained in it will condense. Should expansion lower further, to below the freezing point, the temperature of the compressed, ambient air, the condensed water freezes. In sufficient quantities, the resulting ice restricts flow through the system and decreases performance, possibly to the point where the system becomes inoperable.
Many prior art systems employ one or both of two techniques to ensure that no ice forms that might clog the system. The first of these approaches is to simply design the turbine such that temperature of its outlet air remains above the freezing point. Not only is it then impossible for ice to form, but the size of the heat exchanger, a bulky component accounting for a significant percentage of overall system weight, may be reduced. However, systems of this nature require far more energy to produce a desired amount of cooling than systems in which turbine outlet air temperature is allowed to fall below the freezing point.
The second approach taken in these systems is to operate the turbines below the freezing point and provide the system with the capability both to sense the presence of ice and to deliver warm deicing flow to the regions where an unacceptable level of ice accumulation is indicated. The benefit of this type of system is that the deicing mechanism is operational, and therefore extracts energy from the system, only when ice is detected. Delivering warm deicing flow, however, requires additional hardware that increases the overall weight of the system. In U.S. Pat. No. 3,177,679, when thermostats in the outlets of each of two turbines indicate temperatures below freezing, valves in ducts connecting the turbine outlets with warmer air sources open. In U.S. Pat. No. 4,127,011, a plenum encases the outlet of a turbine. When the temperature within that turbine outlet falls below freezing, valves open to deliver warm air into the plenum, preventing ice from accumulating on the inside surface of the turbine outlet.
An alternative to this second approach is to operate system turbines below the freezing point and mix a continuous flow of warm air with the turbine outlet air to raise its temperature. In U.S. Pat. No. 3,877,246, a system with two turbines employing this technique is described. The outlet air of the first turbine mixes with warm air both recirculated from the cabin and compressed, enabling it to operate below the freezing point. This mixture then expands in a second turbine. Before entering the cabin, the outlet air exhausted from this second turbine passes first through a precipitator to remove any entrained water vapor. To maintain the temperature of air downstream of the second turbine above freezing, a valve in a duct connecting the inlet of the second turbine to the outlet of the second turbine is modulated. A similar system, but with a single turbine, is described in U.S. Pat. No. 2,628,481. Recirculated cabin air is first filtered and then split. The first half of this split, recirculated air mixes directly with the air exiting the turbine. Water vapor entrained in this mixture is then removed in a water separator. The flow exiting the separator then mixes with the second half of the recirculated cabin air before entering the aircraft.
U.S. Pat. No. RE32,100 (reissue of U.S. Pat. No. 4,209,993) and 4,430,867 both describe single-turbine systems that also use the heat contained in recirculated air to maintain the temperature of air downstream of the turbine above the freezing point. Before entering the turbine inlet, compressed supply air first passes through the warm path of a primary condenser, removing entrained water vapor. The dehumidified air exiting the warm path of the condenser is then expanded in the turbine. In U.S. Pat. No. RE32,100, the outlet air exiting this turbine then mixes with warm cabin recirculation air and passes through the cold path of the condenser. In U.S. Pat. No. 4,430,867, the outlet air exiting the turbine passes first into the cold path of a heat exchanger before entering the cabin. Fluid passing through the warm path of the heat exchanger passes first through the cold path of a secondary condenser located in the cabin. Recirculated air is drawn through the warm path of this secondary condenser, dehumidifying it before passing it back into the cabin. The fluid, warmed in the cold path of the secondary condenser, passes subsequently to the cold path of the primary condenser before circulating back to the heat exchanger.
The systems disclosed in both U.S. Pat. No. RE32,100 and 4,430,867, by providing means for the removal of water vapor from the air stream prior to expansion within the turbine, allow the turbine to operate at more efficient subfreezing temperatures. However, these systems fail to recover the heat of vaporization yielded when water vapor is condensed from the turbine inlet stream, contributing to an overall loss of cycle efficiency and cooling capacity.